Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a zoom lens suitable as an image pickup optical system to be used in an image pickup apparatus, such as a monitoring camera, a digital camera, a video camera, and a broadcasting camera.
Description of the Related Art
In recent years, as an image pickup optical system to be used in an image pickup apparatus, a zoom lens is required to have a high zoom ratio and a small overall system size. For example, as an image pickup optical system for a monitoring camera, the zoom lens is required to have a small overall system size and a high zoom ratio, and is also required that favorable optical characteristics can be obtained in imaging during daytime and at night.
In general, a monitoring camera uses visible light in imaging during daytime, and uses near-infrared light in imaging at night. The use of near-infrared light provides an advantage in that imaging can be carried out with less influence of scattering than when visible light is used, for example, in a dense fog with low visibility. Thus, it is demanded that the zoom lens to be used in a monitoring camera be corrected for an aberration in a broad wavelength range from a visible range to a near-infrared range. Hitherto, there is known a zoom lens having a high zoom ratio, which is corrected for various aberrations across a visible range to a near-infrared range.
In Japanese Patent Application Laid-Open No. 2011-053526, there is disclosed a zoom lens having a zoom ratio of 18.73. This zoom lens includes, in order from an object side to an image side, first to fourth lens units having positive, negative, positive, and positive refractive powers, and an interval between adjacent less units is changed during zooming.
In Japanese Patent Application Laid-Open No. 2013-88792, there is disclosed a zoom lens having a zoom ratio of 2.44. This zoom lens includes, in order from an object side to an image side, first to third lens units having positive, negative, and positive refractive powers, and an interval between adjacent lens units is changed during zooming.
In Japanese Patent Application Laid-Open No. 2013-171207, there is disclosed a zoom lens having a zoom ratio of 5.92. This zoom lens include, in order from an object side to an image side, first to fifth lens units having positive, negative, positive, positive, and positive refractive powers, and an interval between adjacent lens units is changed during zooming.
In the zoom lens for a monitoring camera, near-infrared light is used in most cases in imaging at night. However, there are cases where a sufficient amount of light cannot be obtained from the near-infrared light, for example, when there is very little moonlight around the time of a new moon and when the moon is hidden by a cloud. In the meantime, light called nightglow (peak wavelength of 1.6 μm) is emitted when hydroxide ions in an atmosphere are excited by the sunlight. With the use of this light, favorable imaging can be achieved with ease even when there is no moonlight.
In the zoom lens for a monitoring camera, in order to obtain favorable optical characteristics over a broad wavelength range from a visible range to a near-infrared range while achieving a higher zoom ratio, it is important to appropriately set the zoom type and the lens configuration of each lens unit. For example, in a zoom lens that includes three or more lens units including, in order from the object side to the image side, first to third lens units having positive, negative, and positive refractive powers, it is important to appropriately set materials for the lenses configuring the first lens unit or the lenses configuring the second lens unit.